1. Field of the Invention
This invention relates to a compact microwave filter filled with a dielectric and more particularly to a dielectric waveguide-type filter which enables high-pass characteristics, bandpass characteristics and band-rejection characteristics to be selectively realized with a simple structure.
2. Prior Art Statement
As a dielectric waveguide-type filter of the aforesaid type there is known the coaxial dielectric filter. As might be expected from the fact that it is generally referred to as a "ceramic filter," the coaxial dielectric filter uses ceramic as its dielectric material and specifically uses a rectangular block of such material. Five surfaces of the dielectric block, namely the four side surfaces and one of the two end surfaces in the axial direction (that is to say all of the surfaces except one end surface), are coated by vapor deposition or direct application with silver, aluminum or some other appropriate electrically conductive material so as to form conductive layers or conductive films on these surfaces.
The axial length of the dielectric block is equal to one-quarter of the wavelength at the frequency at which the filter is to be used, and the interior of the block has a through-hole passing therethrough in the axial direction. The inner wall of the through-hole is also coated with a dielectric material in the aforesaid manner. As a result, the coaxial dielectric filter constitutes a coaxial dielectric path having a center conductor which measures one-quarter wavelength and is open at one end and shorted at the other. It thus operates as an LC resonator.
The coaxial dielectric filter of this type is troublesome to fabricate because it requires formation of the axial hole through the interior of the dielectric block and, moreover, requires the center conductor to be completed by applying a dielectric material to the inner wall of this through-hole. Another problem is that it is extremely difficult to fine-adjust the characteristics of the completed filter by, for instance, trimming the center conductor. Further, where it is desired to construct a multi-stage filter circuit by cascade-connecting a plurality of such coaxial dielectric filters, the interconnection of the stages has to be carried out by using capacitors to connect the open surfaces of the center conductors of each adjacent pair of coaxial dielectric filters. The connection of the capacitors to the individual filters not only is troublesome work which reduces productivity but is also undesirable from the point of obtaining uniform characteristics. Actual application of this method has often been found to result in characteristics that leave much to be desired and, moreover, since the propagation loss tends to become large, it has been difficult to realize a high Q factor. Worst of all, there has been no simple way to select the filter characteristics to be obtained. Specifically, it has not been possible by simple modifications in the course of fabrication to obtain the desired high-pass characteristics, bandpass characteristics, band-rejection characteristics or the like.
Further examples of conventional dielectric waveguide-type filters are disclosed, for instance, in U.S. Pat. No. 4,691,179 issued to Stephen C. Blum et al. on Sept. 1, 1987 and in U.S. Pat. No. 4,607,242 issued to James C. Cozzie on Aug. 19, 1986. However, the disclosures do not provide techniques for substantially achieving a satisfactory degree of size reduction and the filters disclosed all require complex structures and complicated procedures for adjustment of the characteristics.